Transmission line measuring apparatus



Oct. 31, 1950 o. M. WOODWARD, JR

TRANSMISSION LINE MEASURING --API"ARATUS 2 Sheets-Sheet 11 Original Filed April 25, 1945 lnventor 0alrleyM Madwa (Ittpru Oct. 31, 1950' o; M; WOODWARD, JR 2,527,979

TRANSMISSION LINE MEASURING APPARATUS Original Filed April 25, 1945 2 Sheecs-Sheet 2 LINEAR 057.5670, I/VDIC'AITOK cam Gttormgl Patented Oct. 31 1950 UNITED STATES PATENT OFFICE TRANSMISSION LINE:M:EASURIl\l-G APPARATUS. I a oakiey' M; Woodward, Jr., Princeton, N.- .i'.-, aasigrior to Radio Corporation of America; a corporation ofDelaware r Crigindlapplieation April 25, 1 945, sard ne.

590,27 1. Divided aiid this-application July .10, 1946, Serial Noicsas io This applioation is a division of my copending U. S.--=a'p'plica'tion Serial NI.590,-2'Z.1, filed.April 25, 1945, entitled Transmission Line Measuring Apparatus and assigned-tothe same assignee as 6 Claims (e1; its-44 upon-the type-of measurement *to be made, as will' be described greater detail hereinafter.- The-structure may be readily modified for- 'measurements -on an open-wire line.

the instant application. 5 Amon "the *objects of thexinvention are to This invention relates generally to transprovide an improved method of and means for mission line measuring apparatus and more parmeasuring-the transmission of energy through a ticularly to improved :reflectometers for indicattransmissionline connecting a generator to a ing load matching, load-impedanc and power load. 'Anothero-bject-ofthe inventionis-to prodelivered to a load through a coaxial or open- 10 videan improved refiectometcr for measuring wire transmission line. the degree 'ofmismatchof a lead connected to' TCustomary procedure in matching coaxial a coaxial transmission'line. An additional obtransmission lines "to a load, in measuring load ject of the invention-is-toprovide an improved impedance or in determining the power transreflectometer for :measuring the impedance of :a mitted tosaid -l'oad has been 'to employ aslotted' load connected toacoaxialtransmission line. A sectionbf coaxial line -and a sliding-probe in- 1 further object of the invention is to provide an dicator. Although this method is quite satiS- improved refiectometer for measuring the ourfactory, it is essential that the operator shave rent transmitted to .a eload through a coaxial some knowledge-oftransmission:linetheory-and transmission circuit. .A still further object of practicein' order that such measurements-may the invention is to provide an improved device he re'adilymade- In 'the'case of a load :having for measuring the reflection coeffic'ient or the se'verahadiustable elements, the measurement standing wave ratio in a coaxial transmission and matching process nray be quite complicated. line connecting a highrlfirequency generator to a Si'nce the slotted line and movable ipr'obelappaload. Another object to provide an improved ratus are-primarily laboratory fequipment, they evice for measuring the power transmitted aretl'ldtlwlhSliited forofield measurements, The through a coaxial transmission line connecting instant inventioncomprises a simple Tjunction aegenerator to a load. o al Obj t is (if-coaxial line having one or morecoupling loops o provide an mp v d r m r mpr in selectively inductively :coupled to and -capaci-. a .T Junction of three coaxial transmission lines, tively shielded fromthe'conductors ofiithe line T and atleast: uplinglo p ductively upled junction. v to said T junction for measuring the energy .A;first embodiment of the invention permits characteristics .in atransmission lineconnectin'g the measurement of. the degree of load imped a generator to an unknown load and to a matched ance .mismatch to the transmission. line. ,"A load. second embodiment of .the invention permits The invention will be described in greater measurements 'ofjload matching, load impedance, 30 detail by reference to the accompanying drawloadlcurrent', reflection coefficient and standing ings of which Figure 1 is a schematic circuit wave ratio characteristics of the system. A' third diagram of a first embodiment of the invention, embodiment of the invention permits additional Figure 2 is a schematic diagram explanatory of measurements-of load ;power. second and thirdembodime'nts of the invention, Fundamentally, the-several embodiments of 40 Figure 3 is a schematiccircuit-diagram of the the instant invention comprise current c'ompalidetector and the indicatorportions of the emson-systems in which the current in-theload-line bodiment of the invention adapted to provide and matched line is comparedtothe current in power measurements, Figure 4 is a fragmentary the generator line which is connected through cross-sectional viewof a-nrstembodiment of the the I junction to both the load line and the 4 invention for measuring the degree of load matched line. One or more current. pickup matching Figure 5 is a planiviewof:the.electroloops are symmetricallyplaced with respect to static shield comprising an elementofthe device the T junction to coupl selectively magnetically shown in Figure 4, Figure 6 isa cross-sectional to ;three coaxial lines which are connected review of a modification-.of-,-said,;first embodiment 'spectively to the generator, the matched line, .50 of the invention, Figure "7 is-a plan view of a and the load line. The matched line is matched second embodiment of the invention, Figure .8 with aknown resistorhavinga 'value'equivalent is a cross-sectional, elevational view taken along to'theline surgeimpedance. The ,couplingloop the section line VI-IIe-VI'II :of said second emor loops may be fixedor rotatable and are coupled bodiment of the invention, Figure 9 is an en- .to theT junctionin difierentzmanners depending larged, fragmentary,=cross-sectional view taken "th efpickup loop 9.

along the section line IXIX of said second embodiment of the invention, and Figure 10 is a cross-sectional View of a third embodiment of the invention. Similar reference characters are applied to similar elements throughout the drawings.

Referring to Figure 1, a high frequency generator l is connected through a generator coaxial line 3 to a T junction with two other coaxial lines 5, I which are connected, respectively, to a matched resistor Z and to a load. A coupling loop 9 comprising a single turn is symmetrically coupled to the branch coaxial lines 5 and I at the T junction of the three lines 3, 5 and 7. The coupling loop and the centers of the conductors forming the T junction are in a common plane. Zero current will flow in the pickup loop 9 when the load current IL and th matched line current Im are equal and in phase. This condition obtains only when the load impedance equals the matched line impedance Zc since the two branch lines 5 and i are fed by a common voltage E at the T junction. The matching resistor Z0 is assumed to match exactly geflsurge'ihpedance of the coaxial line 5./Ftf any other load impedance, a result current will be induced in This current may be rectified and indicated by means of a detector and a D.-C. meter, not shown, to indicate the degree of mismatch of the load impedance to the surge impedance of the load transmission line 1.

Assuming a mismatched load, a standing wave will be produced as shown in Figure 1.

The impedance of the load line at the junction Obtaining the impedance of ZB in terms of ZA and where p is the length in electrical degrees from a voltage minimum of the load line I to the T junction; and ZA is the voltag minimum; occurs:

21 +125 an P where R is the standing wave ratio on the load line. Therefore g R+j tan p Z 1+jR tan p Substituting in Equation 3:

" +J' tan P) (R +1 n The current (Io) induced in the pickup loop is proportional to the difference of the two currents f 'j' 1' f llm IL and Im.

(RH p) Assuming a constant standing wave ratio, the pickup loop current will vary as a function of the relative position of the standing wave with respect to the T junction. "For this condition the Wye Wilt vary'lrom a maximum of Q1 7 R for =0 to a minimumottl R) for min. max.

Hence the ratio of the minimum current to the maximum current for a constant standing wave R (ll) ratio and a variable standing wave shift is seen to be equal to the standing-wave-ratio.

For simplicity, a fixed crystal detector, not

shown, may be employed as the rectifier in the pickup loop circuit. Therefore, the meter deflection will be proportional to the square of the pickup loop current. Since a constant input voltage E may be assumed, it is seen that, as the load impedance approaches a match with the surge impedance of the transmission line, the rate of change of the indicating meter deflection rapidly diminishes. In actual practice, the load line I may be matched with adjustable elements such as inductive stubs, each stub being adjusted in turn for minimum meter deflection until the meter provides null or substantially zero indication. Although the exact standing wave ratio of a mismatched load cannot be obtained directly with this embodiment of the invention, an experienced operator may estimate quite accurately standing wave ratios in the higher range for fixe generator power output.

A second embodiment of the invention is illus-- trated schematically in Figure 2, wherein the T junction formed by the three coaxial lines 3, 5 and l is coupled to a rotatable coupling loop disposed in a plane SS normal to the common plane through the three coaxial lines forming the T junction. The plane of the coupling loop may be rotated through an angle of 90 to the position D-D. If desired, as explained in greater detail hereinafter, two separate coupling loops may be employed, one being disposed in the plane SS and the other being disposed in the plane D--D. The two coupling loops would be both magnetically and electrostatically shielded from each other. Lines through the centers of the coupling loops would coincide with the center of the T junction.

Considering first the embodiment of the in vention employing a single rotatable coupling loop, in the position S-S the loop is coupled substantially only inductively to the generator coa xfaiiineiz 'Tiretoupnng'is'sebstanneuypure y inductive" since the loop "is eeamstaticauy shielded frohi'thecoaxiallineby slot's ii'i'tl'ioiitei conductor of theliiies; whichwillbedesciibed' in greater detail by reference to the" structure of Fi'g'uleS "I, 8 93169. When the loopis intheplane' D'D,' it' is inductively coupled slibstahtially'ohly'" to the matched li'n'e" 5and the load line "I. when thecoupling'loopisin the plane'DD; a current In" isinduced in the l'oo'pwhich' is proportional to the vector difierence of the load cu'r'r'eritIB and the inatche'dli'n'e cu'r'reiit'lm. When t V I is in the-plane S-S, a current Is is 'inducedin'tlie loopwhich is proportional to the'vectorsum'of where RB and Xsar'ethe resistive andrea'ctive components of the load impedance Zia:

The ratio of the absolute magnitudes of the loop currents in the planes D-D and S-S is The general transmission line equation is T 7 ZB HBW f Z tjflfi 6 p 17 in operation of the device, if the coupling loop-9' is connected to a linear detector,- and the "linear detector is connected to a suitable D.-C. indicator, the ratio of the rectified loop currents provides the reflection coefficient K. The indicating meter may be calibrated in terms of the standing wave ratio R on the load line I, since R equals The gain of the detector or the power output'o'f the generator may be adjusted to provide full s'cale'reflection of the indicator when-the loop is in theplane SS. Then by rotating the loop'to theplaneD-D, the standing wave ratio R-may be read directly on the meter scale.

The alternative arrangement wherein two loop'srespect to the arrangement of Figure 1.

whereinN is a proportionality constant I areempioyedi onein theplaf s d the; otliri in the plane D-j-D, mayui'ti'lifieaiSiiig1edetector and indicator which iflay be switched to either loop, or separate detectorsand indicators may be used. Since the two loops must be magnetically and electrostatically shielded from each other, the most convenient arrangement is to locate them on opposite sides of the T junction and to shield them by means of a magnetic shield disposed in the plane of the T junction, as is shown in Figure 10.

Although the reflectometer is substantially independent of frequency (assuming "that'.,-the; matched resistor is matched at all operating "ire quencies) the physical size of the coupling loop or loops must be taken into consideration. If the" loop is wide enough" or the frequency sufficiently high, the loop' current will bean integration of the varyingline currents produced by mismatch of the load, and will-not indicate tlie loa'dcii'r rentsfiowing only at the 'I; junction;

It is noted that when the loop is in the plane DD, the device operates in "essentially the same manner as that described "heretofore with However, by providing the rotatable coupling loop, or 1 The [power indicating meter 15 may be caliv brated by applying-known values-oi-powerto the load. I Again referring to Figure-2;, -the absolute inag-' nitude-and the phase angle of the load impedance may be determined-by deriving the loop currents in tWoadditional planes A+A and C C which are disposed at 45 angles with respect to the conductors-forming the T junction. The current- Io in the coupling loop when it is disposed in the plane C- C is- 1 e e since components of the load current IBfiowin opposite directions when the loop is in'the plane since components of -the matched line currentrltt.

Q I p (it 7 flow in opposite directions p when the coupling loop is in the plane A-A E m-Z B ZB f 213 10 1B Ii Hence the ratio of the 100p currents in the two planes -0 and A-A provides the absolute magnitude of the load impedance in terms of the line characteristic impedance. Since Is=Um+IB cos +i Ius sin where is the phase angle of the load impedance It should be noted that the sign of the phase angle is not obtained by these measurements.

Figure 4 shows one embodiment of the invention which provides means for indicating the degree of mismatch of the load impedance. The coaxial lines 3, 5 and l are provided with conventional connectors, not shown, for connection to the generator line, the matching resistor and the load, respectively. An aperture is provided in the outer conductor 2 l' of the lines 5 and I adjacent to the T connection with the generator line 3. A short conductive tube 23, set into the T telescopic inner conductor 3|, the longitudinal penetration of which is controlled by means of a control knob 33.

The quarter-wave resonant line 29 is coupled to the coupling loop 21 adjacent the short-circuited end of said line, thus providing'high sensitivity and selectivity. The pickup loop 9 is coupled into the resonant line 29 at another point near the shorted end of the line. A crystal detector, or other high frequency detecting device, 35, is connected in series with the pickup loop 9 and a D.-C. indicating device 37. A bypass capacitor 39, connected across the indicator circuit adjacent the detector 35, bypasses the alternating components derived fromthe detector. If desired, amplification may be provided in the line connecting the detector and bypass capacitor to the indicator 31.

In operation, the cavity resonator 29 is adjusted until maximum sensitivity is provided at the operating frequency. The matching resistor 8, connected to the matched line 5 may comprise a non-inductive resistor equal to the surge ini-- pedance of the generator line, and mounted within a conventional connector plug inserted into the matched line conductor. In operation the adjusting elements of the load and the load line are adjusted separately to provide minimum indications on the indicator 3?. When all load and load line tuning elements are properly adjusted, a null reading should be provided on the indicator.

Figure 5 illustrates the construction of the electrostatic shield 25 which is interposed between the coupling loop 21 and the T junction. The shield may comprise a circular bezel 4i supporting only one end of the group of parallel disposed wires 43 which extend to within a short distance of each other at a line through the center of the bezel.

The structure of Figure 6.is similar to that of Figure 4 with the exception that the coupling loop 21, and the tubular member 23 surrounding it, have been omitted. The bezel 25 is inserted directly adjacent the aperture 43 in the outer conductor wall 21 adjacent the T junction. The

pickup loop 9 comprises a short, flat metallicstrip supported by a grounded terminal 45 and contacting the end terminal of the tubular crystal detector 35. The crystal detector 35 is enclosed within a connector plug element 41 which includes structure forming the bypass capacitor 39. The connector 41 provides means for connecting the detector through a coaxial line to the indicator at a remote point. Conventional connectors 49, 5| and 53 are provided in the three coaxial lines 3, 5 and 1, respectively, at short distances from the T junction for connections to the generator, matched resistor and loads. respectively.

Figures '7, 8 and 9 show the construction of the second embodiment of the invention wherein the coupling loop is rotatable in a plane normal to the plane of the coaxial lines 3, 5, l, forming the T junction. The coupling loop 9 comprises a single loop of wire supported in an insulating block 55 which is rotatable by means of a control shaft 51 within a bearing formed by means of a shoulder 59 supported by the frame 6] which is clamped to the coaxial lines at the T junction. The coupling loop 9 is brought out to a grounded terminal 63 and an ungrounded terminal 55 which may be connected in any desired manner to a linearv detector 6'1 which is connected to a D.-C. indicator 69. Inductive coupling, but substantially perfect electrostatic shielding, between the inner conductors of the coaxial lines and the coupling loop, is provided by means of slots 'H cut in the sides of the outer conductors of the coaxial lines 3, 5, I, immediately adjacent the T junction.

The control shaft 57 may include a stop element [3 which cooperates with two fixed stops l5 and H to permit rotation of the coupling loop only within an angle of 90 to provide for alternate coupling to either the generator line or to the load and matched lines, for providing indications of load matching, load current, reflection coefiicient or standing-wave-ratio, as described heretofore.

If the system is to be employed for measure ment of absolute load impedance or load phase angle, the stops should be located to permit the coupling loop also to be oriented in the planes A-A and CC, asdescribed with reference to,

Figure 2.

The structure of Figure shows a third embodiment of the invention which is a modification of the structures of Figures 7, 8 and 9, wherein a second coupling loop [9 is disposed on the opposite side of the T junction from the first coupling loop 9. The coupling loops may be connected to the same detector and indicator by simple switching means, not shown, or they may be connected to separate detectors and indicators as explained heretofore.

In order to provide magnetic shielding between thecoupling loops 9 and i9, a fiat shielding member 19 is interposed between the outer conductors of the three coaxial lines 3, 5 and l in the space adjacent the T junction enclosed within the frame 6|. The load and matched line outer conductors are slotted adjacent the T junction on the side of the shielding member I9 adjacent to the first coupling loop 9. The generator line outer conductor is slotted on the opposite or underside of the shielding plate 19 adjacent the T junction. Thus, the first coupling loop 3 is in-. ductively coupled to and capacitively shielded from the load and matched lines 5 and l, and the second coupling loop leis inductively coupled to and capacitively shielded from the generator line electrostatically and magnetically shielded from each other. Thus, two fixed coupling loops are disposed at right angles for obtaining the load power measurements described heretofore.

Thus, the invention described comprises several embodiments and modifications of an improved reflectometer for indicating load matching, absolute load impedance,.load phase angle, load power, reflection coefficient and standing- Wave-ratio in a line connecting a generator to an unknown load.

I claim as my invention:

1. An energy detecting device for a coaxial transmission line including a plurality of sections of coaxial line forming substantially a T junction, means for connecting said transmission line to one of said line sections, means for connecting a load to another one of said line sections, means for connecting an impedance element substantially matched to the surge impedance of said transmission line to the remaining one of said line sections, a rotatable coupling loop selectively and simultaneously coupled to said line sections r at said junction, and means for connecting energy detecting means responsive to currents induced in said coupling loop.

2. An energy detecting device for a coaxial 7 transmission line including a plurality of sections 3. However, the coupling loops 9 and 99 are both coupled to and capacitively shielded from said line sections at said junction, and mean for connecting energy detecting means responsive to currents induced in said coupling loop.

3. An energy detecting device for a coaxial transmission line including a plurality of sections of coaxial line forming substantially a T junction, means for connecting said transmission line to one of aid line sections, means for connecting a load to another one of said line sections, means for connecting an impedance element substantially matched to the surgeimpedance of said transmission line to the remaining one of said line sections, a rotatable coupling loop pivoted adjacent said junction and rotatable in a plane parallel to the common plane of said line sections selectively, and simultaneously inductively coupled to and capacitively shielded from said line sections at said junction, and means for connecting energy detecting means responsive to currents induced in said coupling loop.

4. An energy detecting device for a coaxial transmission line includin a plurality of sections of coaxial line forming substantially a T junction, means for connecting said transmission line to one of said line sections, means for connecting a load to another one of said line sections, means for connecting an impedance element substantially matched to the surge impedance of said transmission line to the remaining one of said line sections, a rotatable coupling loop pivoted adjacent said junction and rotatable between two predetermined quadrantal positions in a plane parallel to the common plane of said line sections selectively and simultaneously inductively coupled to and capacitively shielded from said line sections at said junction, and means for connecting energy detecting means responsive to currents induced in said coupling loop.

5. Apparatus as claimed in claim 3 characterized in that said inductive coupling and capacitive shielding is provided by circumferential slots in the outer'conductors of said line sections adjacent to said junction.

6. Apparatus as claimed in claim 2 characterized in that said inductive coupling and capacitive shielding is provided by circumferential slots in the outer conductors of said load line and impedance line sections adjacent to said junction.

OAKLEY M. WOODWARD, JR.

REFERENGES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

